The present invention relates generally to vascular devices and more particularly, to expandable intraluminal vascular grafts that are usually referred to as stents. The invention is directed to the differentiation of the structural stiffness of such a device, including for example the localized variation of the stiffness of specific portions or elements of a stent.
Stents are used to maintain the patency of vessels in the body. They are typically advanced through the vasculature to the deployment site while in a contracted state where they are caused to expand so as to engage the vessel walls and allow the flow of fluid there through. Such stent can be moved along a guide wire previously positioned in the vessel and then expanded by retraction of a restraining sheath within which the stent is disposed. Subsequent removal of the deployment devices along with the guide wire leaves the stent in place and locked in its expanded state.
The prolonged presence of a foreign body within the bloodflow can, under certain circumstances, lead to a restenosis which would diminish and eventually vitiate the utility of the implanted stent. A restenosis could require cleaning of the stent, replacement of the stent or more drastic surgical intervention. It is of course desirable to minimize the occurrence of restenosis.
A number of different conditions have been found to effect the rate and the extent of restenosis. One such condition is prolonged or repeated trauma to the vessel tissue. In order to minimize such trauma, it is important to ensure that the stent engages the vessel walls very uniformly such that the supporting forces are uniformly distributed. This is often difficult to achieve at the extreme ends of the stent where the abrupt termination of the stent naturally provides for an uneven distribution of supporting forces. Additionally, the overall longitudinal stiffniess of the stent may cause the ends of the stent to become excessively embedded in the vessel walls in the event a linearly shaped stent is forced to conform to a non-linear deployment site. A similar problem arises when the deployment site is tapered or extends across a bifurcation. A uniformly stiff stent which is configured and dimensioned to adequately support such vessel in the large end of the taper or the large portion of the bifurcation may exert excessive forces on the vessel walls in the small end of the taper or the smaller portion of the bifurcation. Any of the described conditions may cause sufficient trauma to be inflicted upon the vessel walls to induce restenosis.
Another identified cause of restenosis is the disruption of the flow of blood. Turbulence caused by such disruption can trigger any number of defense mechanisms by which the body reacts to such unnatural condition. The ends of a stent may again be the cause of such undesirable situation. Any non-conformance to the vessel walls at the extreme ends of the stent could cause portions thereof to project into the bloodflow and thereby cause a disruption. Alternatively, a stent may conceivably distort a lumen sufficiently to disrupt the smooth flow of blood there through.
Efforts to address these problems have previously focused upon variably configuring and dimensioning the struts and spines of the stent in order to reduce their stiffness near the ends of the stent and/or to reduce longitudinal stiffness while maintaining substantial radial strength. For example, by selecting the dimensions of the spines to be relatively smaller than the dimensions of the struts, the desired reduction in stiffniess and strength may be achievable. Alternatively, the width and/or thickness of the various structural components near the ends of the stent may be reduced relative to the width and thickness of the corresponding structural components near the center of the stent in order to decrease the relative stiffness of the stent ends. A similar approach could be taken to adapt a stent to a tapered or bifurcated deployment site. However, such approaches tend to substantially increase the complexity and cost of the manufacturing process. Each and every version of a stent would require its own tooling.
It is therefore most desirable to provide a stent configuration by which traumatization of the vessel walls and disruption of the blood flow is minimized. More particularly, a stent is needed having ends that have a reduced tendency to impinge into and become embedded in the vessel walls as well as to project into the blood stream. Moreover, it is most desirable to provide a manufacturing process by which the stiffniess of various components of the stent can be adjusted quickly and easily to suit the requirements of a particular type of application.
The present invention provides a vascular device that exhibits a differentiated degree of stiffness throughout selected portions of its structure. Such vascular device when in the form of a stent overcomes disadvantages associated with certain prior art stents in that it is capable of providing the necessary support to the vessel walls within which it is deployed without exerting undesirable forces thereon. In accordance with the present invention, the stiffness of the stent is readily differentiated throughout its structure in any of a number of configurations. The stent""s stiffness may thereby be reduced in those areas where it is determined that support is less critical than avoidance of the traumatization of the tissue it is in contact with. Such stiffness differentiation is achieved with the differentiated heat treatment of selected portions or elements of the stent after its fabrication. This provides a further advantage in that a single stent structure can thereby be tailored to accommodate the specific requirements of many different types of deployment sites.
The desired differentiation in the stiffness of the stent of the present invention is achieved by the use of a superelastic material in its construction wherein such material undergoes a transition from a relatively soft and malleable phase to a relatively strong and stiff phase as the material""s temperature is raised through a transition temperature. The phase transition is fully reversible. Approaching the transition temperature from below causes the material to become stronger and stiffer while approaching such temperature from above causes the material to become softer and more malleable. Heat treatment of the material serves to shift the transition temperature to a higher temperature. Heat treatment of isolated portions of the stent serves to shift the transition temperature of only those portions to a higher temperature. By shifting the transition temperature of selected portions of the stent closer to body temperature, i.e., to nearer the temperature at which the stent will be maintained after deployment, such portions while tend to be in a softer and more malleable state than those portions of the stent wherein the transition temperature has not been shifted or has been shifted to a lesser degree.
The desired differentiation may be achieved either by subjecting only those portions of the stent which are to be softer and more malleable to an elevated temperature or by subjecting the entire stent to an elevated temperature while maintaining those portions which are to remain strong and stiff in contact with a heat sink. Any of various heat sources can be used to supply the necessary heat energy in order to achieve a certain shift whereby the total heat energy that is supplied is a function of both the temperature and the total time of exposure to such temperature.
In one embodiment, the stent is heat treated such that the end portions of the stent are in a softer and more malleable state than its center portion when the stent is subsequently subjected to body temperature. The center portion of the stent is thereby able to provide the necessary support to the vessel wall while its ends are less likely to become excessively embedded in the vessel tissue. As a result, the risk of restenosis is reduced.
In an alternative embodiment, the stiffniess of only one end of the stent is reduced in order to enable the stent to more uniformly support a tapered deployment site or a deployment site that extends across a bifurcation. The change in stiffness may be relatively abrupt or may be distributed over a significant portion of the stent. By positioning the more malleable end of the stent in the smaller region of the vessel, traumatization of the vessel tissue is less likely. The risk of restenosis is thereby correspondingly reduced.
In a further alternative embodiment, the longitudinal stiffness of the entire stent is reduced without a reduction in its radial stiffness. This enables the entire stent to more uniformly conform to the vessel walls of a non-linear deployment site without compromising the stent""s ability to support the vessel walls. The stent ends are therefore less likely to become excessively embedded in the vessel tissue to thus reduce the risk of restenosis.
The superelastic material preferred for use in the present invention is nitinol. The temperature at which the transition from the relatively soft and malleable martensitic phase to the relatively strong and stiff austenitic phase is completed is commonly referred to as the A(f). A 10xc2x0 C. to 30xc2x0 C. shift in the A(f) can yield a difference in stiffness of up to about 50%.
These and other features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiments which, taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention.